This problem only occurs on the Motorola Dual Tuner HD DVR cable box. If you press a key on the remote, it sometimes takes two or three tries before the box responds, if at all. This same remote works fine on another non-DVR box. Since this started only in the last two or three days, could it be a problem with a software upgrade or maybe the box has just worn out. It's over two years old. I have reset the cable box and changed the batteries in the remote with no luck. Also, the remote does fine controlling my HD TV.
I'm also having this same problem but my box is new! It's been turned off for several hours before being used again and still doing it. Even my 7 year old son is irritated by this. I can't even be bothered to search for anything on tv, granted I'm highly impatient, but we shouldn't have this problem. I decided to go online to look for something to watch, and apparently I can't change the channel on my tv in this manner either.
Sometimes unplugging the DVR for a few seconds and plugging it back in will resolve problems like this. Obviously, the problem is with the DVR rather than the remote. If the problem persists, consider swapping the DVR for a replacement. Either call 1-800-COMCAST or take the DVR and the remote to the proper Comcast facility.